DNS Structure

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Domain Name Service (DNS) is an application layer protocol that is part of the standard TCP/IP protocol suite. Specifically, DNS is a naming service; it obtains and provides information about hosts on a network.

Domain Name Service performs naming between hosts within your local administrative domain and across domain boundaries. It is distributed among a set of servers, commonly known as "name servers," each of which implements DNS by running a daemon called in.named.

Note - The in.named daemon is also called the Berkeley Internet Name Domain service, or BIND, because it was developed at University of California at Berkeley.

On the client's side, DNS is implemented through the "resolver." The resolver is neither a daemon nor a particular program; rather, it is a library compiled into applications that need to know machine names. The resolver's function is to resolve users' queries; to do that, it queries a name server, which then returns either the requested information or a referral to another server.

DNS Clients

A name server running in.named can also run the resolver; therefore, there can be two kinds of clients:

client-only
client/server

A client-only client does not run the in.named daemon; instead, it consults the resolver, which provides a list of possible name serving machines to which queries should be directed.

A client/server is a machine that uses the domain name service provided by in.named to resolve a user's queries. This type of machine may answer its own queries, but not necessarily.

DNS Servers

You implement DNS for a zone on one or more servers. A zone can include two master servers, and may or may not include other servers.

Master Servers

The "master" name servers maintain all the data corresponding to the zone, making them the authority for that zone. These are commonly called "authoritative" name servers. It is recommended that the data corresponding to any given zone be available on at least two authoritative servers. You should designate one name server as the primary master server and at least one as a secondary master server, to act as a backup if the primary is unavailable or overloaded.

The "primary" master server is the name server where you make changes for the zone. This server loads the master copy of its data from disk when it starts up in.named. The primary server may also delegate authority to other servers in its zone, as well as to servers outside of it.

The "secondary" master server is a name server that maintains a copy of the data for the zone. The primary server sends its data and delegates its authority to the secondary server. When the secondary server boots in.named, it requests all the data for the given zone from the primary. The secondary server then periodically checks with the primary to see if it needs to update its data.

A server may function as a master for multiple zones: as a primary for some zones, and as a secondary for others.

A server at the root level of the network is called a "root domain name server." On the Internet, root domain name servers are maintained by the NIC. If a network is not connected to the Internet, primary and secondary name servers must be set up and administered for the root level of the local network.

Caching and Caching-Only Servers

All name servers are caching servers. This means that the name server caches received information until the data expires. The expiration process is regulated by the time-to-live (ttl) field attached to the data when it is received from another server.

Additionally, you can set up a "caching-only server" that is not authoritative for any zone. This server handles queries and asks other name servers who have the authority for the information needed. But the caching-only server does not maintain any authoritative data itself.